The blood flow capacity in subcutaneous adipose tissue in humans remains largely unknown, and therefore the aim of this study was to determine the physiological range of blood flow in this tissue.

METHODS AND RESULTS:

The subcutaneous adipose tissue blood flow (ATBF) was measured in 9 healthy young men by positron emission tomography using radiowater tracer. Subcutaneous ATBF was determined in regions adjacent to knee extensors at rest and during dynamic knee extensor exercise, and with 2 physiological perturbations: while breathing moderate systemic hypoxic air (14% O2) at rest and during exercise, and during intra-femoral artery infusion of high-dose adenosine infusion. ATBF was 1.3±0.6ml·100g(-1)·min(-1) at rest and increased with exercise (8.0±3.0ml·100g(-1)·min(-1), P<0.001) and adenosine infusion (10.5±4.9ml·100g(-1)·min(-1), P=0.001), but not when breathing moderate systemic hypoxic air (1.5±0.4ml·100g(-1)·min(-1)). ATBF was similar during exercise and adenosine infusion, but vascular conductance was lower during adenosine infusion. Finally, ATBF during exercise in moderate systemic hypoxia was reduced (6.3±2.2ml·100g(-1)·min(-1)) compared to normoxic exercise (P=0.004).

CONCLUSIONS:

The vasodilatation capacity of human subcutaneous adipose blood flow appears to be comparable to, or even higher, than that induced by moderate intensity exercise. Furthermore, the reduced blood flow response in subcutaneous adipose tissue during systemic hypoxia is likely to contribute, in part, to the redistribution of blood flow to exercising muscle in a condition of reduced oxygen availability.

Mitochondrial potassium channels have been implicated in myocardial protection mediated through pre-/postconditioning. Compounds that open the Ca2+- and voltage-activated potassium channel of big-conductance (BK) have a pre-conditioning-like effect on survival of cardiomyocytes after ischemia/reperfusion injury. Recently, mitochondrial BK channels (mitoBKs) in cardiomyocytes were implicated as infarct-limiting factors that derive directly from the KCNMA1 gene encoding for canonical BKs usually present at the plasma membrane of cells. However, some studies challenged these cardio-protective roles of mitoBKs. Herein, we present electrophysiological evidence for paxilline- and NS11021-sensitive BK-mediated currents of 190 pS conductance in mitoplasts from wild-type but not BK−/− cardiomyocytes. Transmission electron microscopy of BK−/− ventricular muscles fibres showed normal ultra-structures and matrix dimension, but oxidative phosphorylation capacities at normoxia and upon re-oxygenation after anoxia were significantly attenuated in BK−/− permeabilized cardiomyocytes. In the absence of BK, post-anoxic reactive oxygen species (ROS) production from cardiomyocyte mitochondria was elevated indicating that mitoBK fine-tune the oxidative state at hypoxia and re-oxygenation. Because ROS and the capacity of the myocardium for oxidative metabolism are important determinants of cellular survival, we tested BK−/− hearts for their response in an ex-vivo model of ischemia/reperfusion (I/R) injury. Infarct areas, coronary flow and heart rates were not different between wild-type and BK−/− hearts upon I/R injury in the absence of ischemic pre-conditioning (IP), but differed upon IP. While the area of infarction comprised 28±3% of the area at risk in wild-type, it was increased to 58±5% in BK−/− hearts suggesting that BK mediates the beneficial effects of IP. These findings suggest that cardiac BK channels are important for proper oxidative energy supply of cardiomyocytes at normoxia and upon re-oxygenation after prolonged anoxia and that IP might indeed favor survival of the myocardium upon I/R injury in a BK-dependent mode stemming from both mitochondrial post-anoxic ROS modulation and non-mitochondrial localizations.

AIM: It is an ongoing discussion the extent to which oxygen delivery and oxygen extraction contribute to an increased muscle oxygen uptake during dynamic exercise. It has been proposed that local muscle factors including the capillary bed and mitochondrial oxidative capacity play a large role in prolonged low-intensity training of a small muscle group when the cardiac output capacity is not directly limiting. The purpose of this study was to investigate the relative roles of circulatory and muscle metabolic mechanisms by which prolonged low-intensity exercise training alters regional muscle VO2 .

METHODS: In nine healthy volunteers (seven males, two females), haemodynamic and metabolic responses to incremental arm cycling were measured by the Fick method and biopsy of the deltoid and triceps muscles before and after 42 days of skiing for 6 h day(-1) at 60% max heart rate.

CONCLUSION: The mechanisms underlying the increase in peak arm VO2 with prolonged low-intensity training in previously untrained subjects are an increased convective O2 delivery specifically to the muscles of the arm combined with a larger capillary-muscle surface area that enhance diffusional O2 conductance, with no apparent role of mitochondrial respiratory capacity.

AIMS: To determine the role played by adenosine, ATP and chemoreflex activation on the regulation of vascular conductance in chronic hypoxia.

METHODS: The vascular conductance response to low and high doses of adenosine and ATP was assessed in ten healthy men. Vasodilators were infused into the femoral artery at sea level and then after 8-12 days of residence at 4559 m above sea level. At sea level, the infusions were carried out while the subjects breathed room air, acute hypoxia (FI O2 = 0.11) and hyperoxia (FI O2 = 1); and at altitude (FI O2 = 0.21 and 1). Skeletal muscle P2Y2 receptor protein expression was determined in muscle biopsies after 4 weeks at 3454 m by Western blot.

RESULTS: At altitude, mean arterial blood pressure was 13% higher (91 ± 2 vs. 102 ± 3 mmHg, P < 0.05) than at sea level and was unaltered by hyperoxic breathing. Baseline leg vascular conductance was 25% lower at altitude than at sea level (P < 0.05). At altitude, the high doses of adenosine and ATP reduced mean arterial blood pressure by 9-12%, independently of FI O2 . The change in vascular conductance in response to ATP was lower at altitude than at sea level by 24 and 38%, during the low and high ATP doses respectively (P < 0.05), and by 22% during the infusion with high adenosine doses. Hyperoxic breathing did not modify the response to vasodilators at sea level or at altitude. P2Y2 receptor expression remained unchanged with altitude residence.

The accuracy and reproducibility of transpulmonary thermodilution (TPTd) to assess cardiac output (Q) in exercising men was determined using indocyanine green (ICG) dilution as a reference method. TPTd has been utilized for the assessment of Q and preload indices of global end-diastolic volume (GEDV) and intrathoracic blood volume (ITBV), as well as extravascular lung water (EVLW) in resting humans. It remains unknown if this technique is also accurate and reproducible during exercise. Sixteen healthy men underwent catheterization of the right femoral vein (for iced saline injection), an antecubital vein (ICG injection) and femoral artery (thermistor) to determine their Q by TPTd and [ICG] during incremental 1 and 2-legged pedaling on a cycle ergometer, and combined arm cranking with leg pedaling to exhaustion. There was a close relationship between Td-Q and ICG-Q (r=0.95, n=151, SEE: 1.452 L/min, P<0.001; mean difference of 0.06 L/min; limits of agreement -2.98 to 2.86 L/min), and TPTd-Q and ICG-Q increased linearly with VO2 with similar intercepts and slopes. Both methods had mean coefficients of variation (CV) close to 5% for Q, GEDV and ITBV. The mean CV of EVLW, assessed with both indicators (ICG and thermal) was 17%, and was sensitive enough as to detect a reduction in EVLW of 107 ml when changing from resting supine to upright exercise. In summary, transpulmonary thermodilution with bolus injection into the femoral vein is an accurate and reproducible method to assess cardiac output during exercise in humans.

KEY POINTS SUMMARY: Severe acute hypoxia reduces sprint performance. Muscle VO2 during sprint exercise in normoxia is not limited by O2 delivery, O2 off-loading from haemoglobin or structure-dependent diffusion constraints in the skeletal muscle of young healthy men. A large functional reserve in muscle O2 diffusing capacity exists and remains available at exhaustion during exercise in normoxia, which is recruited during exercise in hypoxia. During whole-body incremental exercise to exhaustion in severe hypoxia leg VO2 is primarily dependent on convective O2 delivery and less limited by diffusion constraints than previously thought. The kinetics of O2 off-loading from haemoglobin does not limit VO2 peak in hypoxia. Our results indicate that the limitation to VO2 during short sprints resides in mechanisms regulating mitochondrial respiration.

ABSTRACT: To determine the contribution of convective and diffusive limitations to VO2 peak during exercise in humans oxygen transport and haemodynamics were measured in eleven men (22 ± 2 years) during incremental (IE) and 30-s all-out sprints (Wingate test, WgT), in normoxia (Nx, PI O2 :143 mmHg) and hypoxia (Hyp, PI O2 :73 mmHg). Carboxyhaemoglobin (COHb) was increased to 6-7% before both WgTs to left-shift the oxyhaemoglobin dissociation curve. Leg VO2 was measured by the Fick method, and leg blood flow (BF) with thermodilution and muscle O2 diffusing capacity (DMO2 ) was calculated. In the WgT mean power output, leg BF, leg O2 delivery and leg VO2 were 7, 5, 28 and 23% lower in Hyp than Nx (P < 0.05), however, peak WgT DMO2 was higher in hypoxia (51.5 ± 9.7) than Nx (20.5 ± 3.0 ml min(-1) mmHg(-1) , P < 0.05). Despite a similar PaO2 (33.3 ± 2.4 and 34.1 ± 3.3 mmHg), mean capillary PO2 (16.7 ± 1.2 and 17.1 ± 1.6 mmHg), and peak perfusion during IE and WgT in Hyp, DMO2 and leg VO2 were 12 and 14% higher during WgT than IE in Hyp (both, P < 0.05). DMO2 was apparently insensitive to COHb (COHb: 0.7 vs 7%, in IE Hyp and WgT Hyp). At exhaustion, the Y equilibration index was well above 1.0 in both conditions, reflecting greater convective than diffusive limitation to the O2 transfer both in Nx and Hyp. In conclusion, muscle VO2 during sprint exercise is not limited by O2 delivery, the O2 off-loading from haemoglobin or structure-dependent diffusion constraints in the skeletal muscle. These findings reveal a remarkable functional reserve in muscle O2 diffusing capacity. This article is protected by copyright. All rights reserved.

AIM:We examined the Fick components together with mitochondrial O2 affinity (p50mito ) in defining O2 extraction and O2 uptake during exercise with large and small muscle mass during normoxia (NORM) and hyperoxia (HYPER).

METHODS:Seven individuals performed two incremental exercise tests to exhaustion on a bicycle ergometer (BIKE) and two on a one-legged knee extension ergometer (KE) in NORM or HYPER. Leg blood flow and VO2 were determined by thermodilution and the Fick method. Maximal ADP-stimulated mitochondrial respiration (OXPHOS) and p50mito were measured ex vivo in isolated mitochondria. Mitochondrial excess capacity in the leg was determined from OXPHOS in permeabilized fibers and muscle mass measured with magnetic resonance imaging in relation to peak leg O2 delivery.

RESULTS:The ex vivo p50mito increased from 0.06±0.02 to 0.17±0.04 kPa with varying substrate supply and O2 flux rates from 9.84±2.91 to 16.34±4.07 pmol O2 ·s-1 ·μg-1 respectively. O2 extraction decreased from 83% in BIKE to 67% in KE as a function of a higher O2 delivery, and lower mitochondrial excess capacity. There was a significant relationship between O2 extraction and mitochondrial excess capacity and p50mito that was unrelated to blood flow and mean transit time.

Maximal oxygen consumption testing is suggested to be regularly included between training blocks of athletes in order to monitor changes in fitness throughout the season. However, despite the good reliability and validity of this physiological test, an expensive metabolic chart, and expert personnel are needed. Further, the maximal effort needed by the athlete makes this test difficult to be performed routinely. Therefore, it is important to develop valid tools that are also feasible for the estimation of the maximal oxygen consumption. The aim of this study was to validate the Ekblom-Bak test (EBT) (Ekblom-Bak et al., 2014) against an incremental test measuring peak VO2 by gas exchange on a cycle ergometer in well-trained individuals.

Methods

33 highly active individuals aged 34.5±6.6yrs (mean ± standard deviation (SD)) body mass 74.5±12kg, and height; 178± 9.3m) participated in the study. The EBT test was performed prior to the incremental exercise test to peak effort on a cycle ergometer for VO2peak assessment. Oxygen uptake was determined by an automated measuring system for oxygen uptake with a mixing chamber (OxygenPro, Jaeger GmbH, Germany) validated against the Douglas bag method resulting in a typical error of 2%. The mean difference and standard deviation of the differences between the EBT and measured VO2peak was calculated with Bland-Altman analysis.

Results

The measured mean and SD VO2peak was 4.1±0.8 L•min-1 for the whole group (male 4.4±0.6 L•min-1 and female 2.9±0.5 L•min-1). The mean differences between measured and estimated (EBT) VO2peak was 0.05 L•min-1 (95% CI; -0.15 to 0.25). CV was 13.2% in the whole group with no significant differences between sexes. For individuals with a VO2peak within the valid range of the EBT (VO2max 1.56 to 4.49 L•min-1, n=23), the mean differences between measured and estimate VO2peak was -0.22 L•min-1 (95% CI; -0.36 to -0.08), resulting in a CV of 8.2%. For individuals above the valid limit (n=10), the mean difference was 0.68L•min-1(95% CI; 0.47 to 0.98) with a CV of 6.9%. Discussion The Ekblom-Bak test is an easily applied and inexpensive screening tool for a population of highly active individuals within the current validity range, and may be used routinely in monitoring fitness.

It is an ongoing debate whether mitochondrial function deteriorates with aging. Some studies demonstrate metabolic dysfunction and increased ROS production (Squier, 2001) while others find no bioenergetics alterations (Rasmussen, et al., 2003). The extent to which exercise maintains or restores mitochondrial function is unknown. Methods: A unique group of elderly lifelong trained males (64±2 yr, BMI=24.5±0.4 kg/m2, VO2max=3.5±0.07 L/min), who maintained road cycling training ~250 km/week 50 years were recruited for the experiment (group ET) along with a group of healthy age matched untrained males (age 65±2 yr, BMI=27±1, VO2max=2.4±0.1), who lived a sedentary lifestyle (group UT). A biopsy was obtained from Vastus lateralis under local anaesthesia. 15-25 mg muscle was immediately prepared and permeabilized for High Resolution Respirometry as previously described (Boushel, et al., 2011). LEAK respiration was assessed by addition of malate (2 mM) and octanoyl-carnitine (0.2 mM). Coupled ADP-stimulated respiration (OXPHOS) of complex I was assessed with pyruvate (5mM) and glutamate (10 mM), and ADP (5 mM). Maximal OXPHOS was determined by addition of succinate (10 mM) for convergent electron input to both complexes I and II. The isolated activity of cytochrome c oxidase (COX), was determined in a redox reaction with TMPD (N,N,N,N-tetramethyl-p-phenylenediamine dihydrochloride) (0.5 mM) and ascorbate (2mM) after blockade of electron flow through complex III with antimycin A (2.5μM) followed by sodium azide (100 mM). Findings: All values are mean±S.E.M. Preliminary findings show that mitochondrial OXPHOS capacity for CI (PI) is substantially higher (50±3 vs. 30±1 pmol.sec-1.mg-1, p<0.0001) in the ET compared to the UT, and maximal OXPHOS with CI+CII (PI+PII) was 2-fold higher (103±3 vs. 51±2 pmol.sec-1.mg-1, p<5*10-5). The higher substrate control ratio (CI+II/CI) in ET (2.05±0.09) compared to UT (1.7±0.06) suggests less substrate competition at the mitochondrial level. These findings indicate that exercise training throughout life induces both regulatory and mitochondrial density changes that optimize mitochondrial function. The observed OXPHOS capacities in the ET subjects are fully comparable to values reported in young elite trained subjects (Jacobs & Lundby, 2013), illustrating no mitochondrial age-related deteriorating in these subjects.

13. Larsen, Steen

et al.

Scheede-Bergdahl, Celena

Whitesell, Thomas

Boushel, Robert

Swedish School of Sport and Health Sciences, GIH, Department of Sport and Health Sciences, Åstrand Laboratory of Work Physiology.

Type I diabetes mellitus (T1DM) is a chronic disorder, characterized by an almost or complete insulin deficiency. Widespread tissue dysfunction and deleterious diabetes-complications are associated with long-term elevations of blood glucose. The aim of this study was to investigate the effects of type I diabetes, as induced by streptozotocin, on the mitochondria in skeletal muscles that predominantly consist of either slow or fast twitch fibers. Soleus (primarily slow twitch fiber type) and the plantaris muscle (mainly fast twitch fiber type) were removed in order to measure mitochondrial protein expression and integrated mitochondrial respiratory function. Mitochondrial capacity for oxidative phosphorylation (OXPHOS) was found to be higher in the slow (more oxidative) soleus muscle from STZ rats when evaluating lipid and complex I linked OXPHOS capacity, whereas no difference was detected between the groups when evaluating the more physiological complex I and II linked OXPHOS capacity. These findings indicate that chronic hyperglycemia results in an elevated intrinsic mitochondrial respiratory capacity in both soleus and, at varying degree, plantaris muscle, findings that are consistent with human T1DM patients.

Editorial. The article discusses research being done on apolipoprotein A-I (ApoA-I). It references the study "High-Density Lipoprotein Maintains Skeletal Muscle Function by Modulating Cellular Respiration in Mice," by M. Lehti et al. published in the 2013 issue of "Circulation." It is hypothesized that understanding the importance of the mechanistic links between high-density lipoprotein (HDL) and energy metabolism would contribute to the prevention and treatment of type 2 diabetes mellitus.